1. Field of the Invention
The present invention relates to a flexible electronic device and a production method of the same. In particular, the present invention relates to a method for producing a flexible silicon electronic device and a flexible liquid crystal display device.
2. Description of the Prior Art
In these years, demand has grown for IC cards provided with a built-in memory circuit and a microprocessor circuit, as they have larger storage capacity compared with magnetic cards. Usually these IC cards are often carried in wallets and the like, and the cards are frequently subjected to bending forces caused by the movements of the carriers. However, conventional IC chips, namely, semiconductor chips themselves formed with silicone wafer are not flexible, and moreover, are relatively fragile, and hence there is a high possibility that these chips will be damaged when external forces are exerted thereon. For the purpose of preventing the damage of such IC chips, for example, Japanese Patent Laid-Open No. 9-312349 (FIGS. 1 to 4, pp. 4 to 10) discloses a procedure in which a semiconductor IC chip formed on a silicon wafer is transferred to a flexible resin sheet. Additionally, Japanese Patent Laid-Open No. 2002-111222 (FIGS. 1 to 3, pp. 3 to 4) and Japanese Patent Laid-Open No. 2002-111226 (FIG. 1, p. 4) disclose multilayer composite boards in which IC chips having various functions are laminated and modules using these multilayer composite boards.
As described above, development of techniques have widely been promoted for actualizing flexible silicon devices and high-function system-in-packages by transferring integrated circuits (hereinafter referred to as IC's) formed by using silicon wafer to resin substrates.
Additionally, in these years, development has been promoted for flexible liquid crystal devices using resin substrates, as thin film transistor liquid crystal display devices are light in weight and resistant to fracture. As a procedure for actualizing the above mentioned devices, a technique has been developed in which a thin film transistor array once formed on a glass substrate is transferred onto a resin substrate. For example, glass substrate on which a thin film transistor array is formed is subjected to wet etching from the back surface of the glass substrate with the aid of a HF based solution to completely remove the glass substrate, and thereafter a resin substrate is adhered to the etched surface to form a flexible thin film transistor board (Akihiko Asano and Tomoatsu Kinoshita, Low-Temperature Polycrystalline-Silicon TFT Color LCD Panel Made of Plastic Substrates, Society for Information Display 2002 International Symposium Digest of Technical Papers, United States, May 2002, pp. 1196 to 1199). Description will be made below on this conventional process on the basis of FIGS. 1A to 1D. A protection sheet 24 is adhered onto the surface of a glass substrate 23 on which an etching stopper 21 and a thin film transistor array 22 are formed (FIG. 1A). Successively, a HF based solution is used to completely remove the glass substrate from the back surface of the glass substrate in such a way that the etching is terminated by the etching stopper 21 (FIG. 1B). A resin substrate 25 is adhered onto the etched surface (FIG. 1C). Finally, the protection sheet 24 is peeled to complete the transference (FIG. 1D). Alternatively, Japanese Patent Laid-Open No. 11-212116 discloses a method in which instead of wet etching, a chemical polishing method is used to completely remove a glass substrate and thereafter a thin film transistor array is transferred on to a resin substrate.
Furthermore, Japanese Patent No. 2722798 discloses a production method in which a liquid crystal display element formed with a pair of glass substrates is soaked in an etching solution to make the glass substrates thinner.
Among the above described conventional techniques, in Japanese Patent Laid-Open No. 9-312349, a step of peeling a semiconductor IC chip from a silicon wafer and a step of transferring onto a flexible resin sheet are low in yield to thereby raise the production cost. Additionally, the IC chip is of the order of a few tens μm in thickness and is not transparent so that the applicable areas thereof are limited and the element isolation of the active elements (for example, transistors) is complicated in such a way that mixed loading of high voltage elements and low voltage elements becomes difficult. The multilayer composite boards disclosed in Japanese Patent Laid-Open No. 2002-111222 and Japanese Patent Laid-Open No. 2002-111226 have semiconductor IC chips, formed of silicon wafer, mounted on resin substrates and thus raise the production costs. Additionally, it has become obvious that these semiconductor IC chips suffer from deterioration in characteristics caused by self-heating.
Furthermore, in the technique in which transferring is made onto a resin substrate after the glass portion has been completely removed from the back surface thereof by etching or polishing a glass substrate on which a thin film transistor array is formed, for the purpose of forming flexible liquid crystal display devices, the step of peeling the protection sheet, illustrated in FIG. 1D, requires a fairly long period of time for successfully transferring the thin film transistor array onto the resin substrate, and hence the throughput concerned is remarkably degraded. Additionally, the step of forming an etching stopper layer as a film is to be added, which raises the costs. Japanese Patent No. 2722798 takes as its object only the reduction of the glass substrate thickness, but does not contain a concept leading to production of flexible devices subsequent to the thickness reduction.